1. Field of the Invention
The invention relates to tractor-trailer subframes and, in particular, to movable subframes for tractor-trailers. More particularly, the invention is directed to a movable subframe for tractor-trailers that eliminates conventional hangers heretofore utilized to pivotally mount suspension assembly beams on the subframe, resulting in reduced stress on the frame and greater wheel stability, as well as a reduction in the number of parts, complexity, weight, and cost of the subframe.
2. Background Art
Movable subframes, typically referred to as slider boxes, slider subframes, slider undercarriages, or slider secondary frames, have been utilized on tractor-trailers or semi-trailers for many years. One or more axle/suspension systems usually are suspended from a single slider box. For purposes of clarity, hereinafter the present invention will be referred to as a slider box. It is understood that a slider box outfitted with usually two axle/suspension systems typically is referred to as a slider or slider tandem, and again, for purposes of clarity will hereinafter be referred to as a slider tandem. The slider tandem in turn is mounted on the underside of the trailer frame, and is movable longitudinally therealong to provide a means for variable load distribution and vehicular maneuverability.
More specifically, the amount of cargo that a trailer may carry is governed by local, state and/or national road and bridge laws, and is dependent on proper load distribution. The basic principle behind most road and bridge laws is to limit the maximum load that a vehicle may carry, as well as limit the maximum load that can be supported by individual axles. A trailer having a slider tandem gains an advantage with respect to laws governing maximum axle loads. More particularly, proper placement of the slider tandem varies individual axle loads or redistributes the trailer load so that it is within legal limits. Once properly positioned, the slider tandem is locked in place on the underside of the trailer by a retractable pin mechanism.
Conventional or prior art slider box designs were developed before the advent of air suspension systems for trailers. At that time, leaf spring suspension systems were the suspension of choice for van trailer frames and slider boxes. However, the leaf spring suspension system was unable to provide much load distribution for varying load situations.
Moreover, the subsequent development of air suspension systems provided load equalization among multiple axles for tractor-trailers, with or without the utilization of slider boxes, as well as improved ride quality for individual axles. Of course, the combination of a movable slider box and an air suspension system provided maximum versatility with respect to variable load distribution, load equalization and maneuverability in a trailer. Unfortunately, prior art slider boxes equipped with air suspensions add unwanted weight to the trailer, primarily because those slider boxes were originally built to support spring suspensions and adapting them to incorporate air suspensions required additional bracing and support. Such additional bracing and support has heretofore included the incorporation of frame brackets or frame hangers into the structure of the slider box as a means for pivotally mounting suspension beams of usually two leading or trailing arm air-ride axle/suspension systems on the slider box.
Thus, within the trucking industry, reducing the weight of carrier equipment without sacrificing durability directly improves productivity by increasing the available payload that can be transported by the vehicle. As a result, the trucking industry continually is striving for additional improvement in slider box design.
The present invention contributes to solving the problem of excessive weight, and improves performance without sacrificing durability in a slider tandem. More specifically, the present invention eliminates conventional hangers of the slider box heretofore utilized to pivotally mount suspension assembly beams of the one or more axle/suspension systems typically suspended from a slider box. This elimination of the structural hangers results in reduced stress on the slider box and greater wheel stability while, at the same time, reducing the number of parts, complexity, weight and cost of the slider box.